Comparative evaluation of PRRSV MLV vaccine efficacy administered via intranasal and intramuscular routes in early age of piglets

Early porcine reproductive and respiratory syndrome virus (PRRSV) infections in suckling pigs are commonly observed, emphasizing the need for early vaccination strategies. However, the effectiveness of early vaccination in suckling pigs remains debated due to potential interference from maternally derived antibodies (MDA) and the immature immune system of piglets. This study evaluated and compared protective efficacy and immune responses of intranasal (IN) and intramuscular (IM) administration of a commercial PRRSV-2 modified live virus (MLV) vaccine in 3-day-old piglets against a Thai highly pathogenic (HP)-PRRSV strain. Piglets from PRRSV-seropositive sows were randomly allocated to four groups: unvaccinated, unchallenged control (A-NEG); unvaccinated, HP-PRRSV-challenged control (B-POS); intramuscular vaccination with PRRSV-2 MLV (C-IM); intranasal vaccination with PRRSV-2 MLV (D-IN). At five weeks of age, piglets in the B-POS, C-IM, and D-IN groups were challenged with HP-PRRSV. Following challenge with HP-PRRSV, both C-IM and D-IN groups demonstrated significant reductions in fever and improved growth performance compared to the B-POS group. Notably, the C-IM group exhibited lower viral loads in the tracheobronchial lymph nodes (TBLN), while the D-IN group showed significantly reduced viral loads in the tonsils. As expected, neutralizing antibody against HP-PRRSV was low across all groups. Interestingly, the D-IN group exhibited enhanced systemic T lymphocyte responses, characterized by increased numbers of PRRSV-specific IFN-γ⁺-producing CD4⁺ and CD8⁺ T lymphocytes following viral challenge. Early vaccination of piglets with PRRSV-2 MLV effectively reduced clinical signs and improved growth performance during HP-PRRSV infection, regardless of the administration route. Notably, intranasal vaccination induced higher PRRSV-specific T lymphocytes responses, highlighting its potential as an alternative strategy for early vaccination in piglets. These findings underscore the importance of optimizing vaccination protocols to improve PRRSV control measures in swine farms, particularly against early PRRSV infections.